Phase Discontinuity Protection; Thermal Overload Protection - ABB REX 521 Technical Reference Manual, Standard Configurations

REX 521
9.1.3.3.
9.1.3.4.
108
Protection Relay
Technical Reference Manual, Standard Configurations
While using the sensitive non-directional earth-fault it is recommended to secure the
earth-fault functionality by using the start signal from a residual voltage relay to
release blocking of the earth-fault protection. By doing this, unnecessary trips are
avoided due to disturbances in the measured earth-fault current.
The settings for the earth-fault current protections can be found in
Main menu\Protection\Io>\ ... \Io>>>\.

Phase discontinuity protection

The phase discontinuity stage Iub> has a tripping function when used for protecting
overhead lines. In cable networks, where phase discontinuity does not cause
dangerous situations, the Iub> stage can give an alarm signal. The phase
discontinuity protection can be used irrespective of the earthing principle.
An example of a phase discontinuity fault is a broken phase wire that has fallen
down on such a place that the resistance towards earth, the specific soil resistivity,
is very high. The earth-fault protection alone is not able to detect the fault and thus
the feeder is not disconnected. The phase discontinuity protection is specially
important in overhead lines and particularly in overhead lines with insulated phase
wires. Another example of a phase discontinuity fault is a line breakdown and a one-
phase earth-fault on the load side. Note that a fault in the end of a long feeder behind
several loads may result in a very low unbalance and thus the phase discontinuity
protection may not operate.
The unbalance of the network is detected by monitoring the highest (IL
lowest (IL ) phase currents. The unbalance is calculated according to the
min
following formula:
IL –
∆I × max
= 100% ------------------------------- -
IL
max
Full unbalance (100%) occurs when a phase conductor is broken before the first load
on the phase. Due to asymmetrical load in the network, the normal unbalance has to
be considered when defining the start value. Since this type of protection cannot be
graded with other systems, it is confined to a supplementary role by the use of a long
time delay, adjustable from 1s up to 300 s.
The settings for the phase discontinuity protection can be found in
Main menu\Protection\Iub>\Setting group 1\, where the
parameters "Operation mode", "Start unbalance" and "Operate time" for the
protection function can be set.
For example, the pulse length for the trip and the start signal can be set in
Main menu\Protection\Iub>\Control setting\.

Thermal overload protection

The thermal overload protection 3Ith> is designed for protection of three-phase
power cables and overhead lines. The calculation of the thermal model is based on
the TRUE RMS measuring principle. The thermal load is calculated by means of the
IL
min
1MRS 751802-MUM
) and the
max